The present invention relates generally to an aluminum alloy food can body and to a method for making such a can body from an uncoated aluminum alloy sheet, preferably a high strength aluminum alloy sheet. Even though aluminum is widely used in beer and other carbonated beverage can bodies, aluminum has heretofore not been extensively used in the manufacture of food can bodies. At least a part of the reason for the difference in usage of aluminum in the carbonated beverage and food can markets is related to differing strength requiremehts for food and carbonated beverage cans, and the effect the differing performance requirements have had on the opportunity to reduce the amount of aluminum needed to make a beverage or food can body.
A carbonated beverage can must be able to sustain an internal pressure from the CO.sub.2 released from the beverage and must have sufficient column strength to withstand the axial load imposed upon the can side wall during double seaming to close and seal the open end of the can. The can must also have sufficient column strength to be reasonably handled and stacked during shipping and storing.
Dunn et al U.S. Pat. No. 3,730,383 is offered as being representative of many patents which describe a typical present-day aluminum carbonated beverage can and the method of making it. Typically, the can body is blanked and drawn into a cup from an uncoated metal stock approximately 0.012 inch to 0.014 inch thick. The cup is then drawn and ironed to reduce the side wall thickness to approximately 0.0045 inch to 0.0047 inch and to size the cup to the desired can body diameter. The bottom end wall is usually formed to a configuration suitable to accommodate the internal pressure, and the can body open end is trimmed and flanged to complete the process. It may be seen that a typical aluminum carbonated beverage can has a side wall thickness of only approximately 34 to 38% of the end wall or the metal feed stock, and thus a substantial amount of weight is reduced in the can and less metal is required to produce the can.
Such a can body or method of making it is not suited for making an aluminum food can body primarily because of the difference in the structural requirements of the food can which, in turn, results in a substantial difference in the side wall and end wall thickness relationship.
While a food can design is concerned with an internal pressure and column strength, as is the carbonated beverage can design, the food can design must also accommodate a negative pressure or vacuum within the can body after it is filled, closed and sealed. This concern for a vacuum in many food can uses results from the processing of the filled food cans. High internal vacuums are used in most cans filled with food products. A common method of food packing is to fill and seal the container and then heat the container in a retort, for example, for a specified time. The pressure inside the container increases as the filled food can is heated. A vacuum may result as the can cools after heating which has the effect of exerting an exterior pressure on the can. The external pressure acting upon the side wall of the can is referred to as a paneling pressure, and failure of the side wall to sustain the pressure is referred to as paneling. The minimum strength requirements of a food can, particularly as those requirements relate to internal pressure and vacuum, will vary with the food product and food processing conditions. Typically, however, in a conventional size aluminum food can, such as a 300.times.407 (3" O.D..times.4 7/16" height), for example, the side wall and end wall will require substantially the same thickness to sustain the internal pressure, vacuum and column strength loads.
In many of the most common and widely used food can sizes and food can uses, the side wall is a critical structural element, and even though it can be stiffened by providing a plurality of annular beads in a central portion thereof, the thickness of the side wall controls or dictates the thickness of the aluminum food can feed stock. Since saving in metal by ironing and thinning the complete side wall is not available in making an aluminum food can, as is the case in making an aluminum carbonated beverage can, aluminum has not enjoyed widespread usage in the food can market.
For some can sizes, however, aluminum has been and is being used in making food can bodies. Small size cans having limited height to diameter ratios can be made from aluminum because the weight of the metal has less impact on the total cost of the can than if the can is large, and further, a precoated sheet can be used to make the can because only a limited amount of forming is required.
Usually a food can body is required to have an organic coating that is compatible with the food to be packaged on the interior of the can to protect the food from contamination. Precoating the aluminum sheet by roller coating, for example, before making small size can bodies has been advantageous because the cost of roller coating has generally been more economical than the cost of applying the coating by methods used heretofore, and further, the roller applied coating provided a higher degree of uniformity and integrity of coating than was available by other methods of coating, such as spray coating, for example.
Typically, precoated aluminum sheet has been used to make such small size food can bodies by drawing and redrawing a blank cut from roller coated coil stock.
Such a method is not economical, however, for making food cans which are larger and in general widespread use. To draw and redraw precoated material in making a food can, larger clearances are required between male and female tooling components to prevent the coating from being scratched, abraded or otherwise damaged. These larger clearances promote metal thickening in the upper section of the can body's side wall adjacent the open end and tapering away therefrom during drawing and redrawing to form the can body. Also as a part of the typical method of manufacture of a precoated aluminum food can body, a flange is provided on the first draw cup and is carried through subsequent redraws until the can body is formed to the desired diameter and height, at which point a marginal edge portion of the flange is trimmed away. It may be seen that this thickening in the upper side wall, flange area and trimmed portion requires an excess of metal to meet design requirements, thus significantly increasing the cost of metal in the can body and offsetting at least some of the advantage in using a precoated feed stock.
A can's cost is of considerable importance, since the cost may approach or exceed the cost of the packaged commodity. Therefore, any cost reduction in the container is desirable. Further, the desirability of aluminum as a packaging material has been well established in the carbonated beverage market. Among other advantages, aluminum is a readily recyclable metal, and its substantial use in the carbonated beverage market has fostered savings of raw materials, energy savings and reduction of litter.
It would be desirable, therefore, to provide an improved and economical method of making an aluminum food can in the most common and prevalent sizes.